1. Technical Field of the Invention
The present invention relates optical MEMS interferometers.
2. Description of Related Art
Micro Electro-Mechanical Systems (MEMS) technology, with its various actuation techniques, enables the realization of new functions and features of photonic devices. For example, by using MEMS actuation to control a movable mirror of an interferometer, displacements in the interferometer optical path length can be introduced, and consequently, a differential phase between the interfering beams can be obtained. The resulting differential phase can be used to measure the spectral response of the interferometer beam (e.g., using Fourier Transform Spectroscopy), sub-surface images of tissues (using Optical Coherence Tomography), the velocity of the moving mirror (e.g., using the Doppler Effect), or simply as an optical phase delay element.
MEMS interferometers are key elements in many sensor applications, such as optical spectrometers. MEMS interferometers have recently been developed using technologies such as surface micromachining, LIGA and Deep Reactive Ion Etching (DRIE) on Silicon on Insulator (SOI) wafers.
Most micromachined interferometers are based on a conventional beam splitting technique using reflection and transmission by a dielectric interface (beam splitter). The interferometer typically further includes fixed and movable metallic mirrors. When using DRIE on SOI wafers technology, MEMS micro-mirrors are formed by selective metallization of silicon side walls using a shadow mask sputtering technology. This metallization technique represents one of the main problems of micromachined interferometers, as it is required to leave a relatively large distance between the mirror and beam splitter. This distance results from a requirement of metallization of the mirror, while keeping the beam splitter dielectric surface protected from metal. Such a long optical propagation distance degrades interferometer performance, especially with limited SOI device height. In addition, absorption losses may be introduced in the near infrared and visible wavelength ranges due to propagation within the medium of the beam splitter.
Another problem encountered in micromachined interferometers is dispersion. As described above, in a MEMS based interferometer, the beam splitter may be a silicon wall or simply the air/silicon (or any other material) interface. In such structures, the optical beams traverse a silicon part in one arm, while the second arm is free from silicon (e.g., propagation in air only). As silicon (or any other equivalent material for the beam splitter) has a refractive index that varies with wavelength, a phase error may be introduced in the interferometer (a phase shift that is dependent on the wavelength). This problem may be addressed by adding compensating interfaces. However, adding such compensating dielectric interfaces may lead to more Fresnel loss if these interfaces are not anti-reflection coated. Anti-reflection coating is not an easy process in optical MEMS monolithic systems, such as those fabricated using DRIE of SOI wafers. Moreover, AR coating is not typically wideband enough for applications like FTIR Spectroscopy. Having many interfaces also leads to more scattering loss due to surface roughness of etched surfaces using DRIE. Therefore, there is a need for a more efficient micromachined interferometer.